How could we choose the plenum box sizing? is there any calculations?

Normally there are no calculations except to adjust velocity to avoid noise.

• If it has been used before grilles then250 -350 fpm air velocity is good choice because this is the same velocity of air in the grille.
• if it is in the mixing box of AHU then it will be more as per design of AHU ranging from700 -900 FPM.
• If it is in the duct for several branches then it will be almost same or plus minus10% as duct air velocity

 

Hope this will work

 

Regards

Maximum acceptable air velocity at plenum box is210-250 fpm grater the velocity of air would carrying the condensation from cooling coil. Design velocity of air  150 fpm is acceptable 

What is a Plenum Box?

 

A plenum is a distribution box attached to the supply outlet and often return inlet of the air handling unit. And the purpose of plenum box is to distribute the cooled or heated air to several spaces. It is also used to balance the air and attenuate the sound caused by air movement in the duct.

To size a plenum box we need to look at the famous equation of Continuity

Q = V x A --------- (1)

Where;

Q = flow rate (Cubic feet per minute) >> CFMV = Velocity (feet per minute) >> ft/min

A = Area of plenum box (Square feet) >> ft2   As per the standards velocity for Air handling unit (AHU) is 800 ft/min, and let say our calculated heat load tonnage is = 8 TR.As rule of thumb 1TR needs 400 CFM So, 8 TR needs = 8 x 400 = 3200 CFM As per equation 1:

• A = Q/V
• A = 3200/800
• A = 4 ft2

Area of Plenum box = W x H --------- (2) According to site condition we have to assume height of plenum box andtry to find width Assume that H = 1.6 ft So, using values in equation 2:   Width of plenum box W = Area of plenum box / height of plenum box = A/H W = 4/1.6 Depth of plenum can be calculated by using following equation: D = 2.5 x d --------- (3)   Where d = diameter of fan blower of AHU Let’s say d = 12” Using equation 3: D = 2.5 x 12 D = 30” = 2.5 ftFor complete detail please CLICK HERE

Depending what application you are referring to! Commercial or Residential! Manuals for Residential: D, S, T, and a heat load calc program. Manuals for Commercial: CS, Q, and a heat load calc program.